Treatment of tinplate



United States Patent TREATMENT OF TINPLATE William David Poole,Baltimore, Md., assignor to Bethlehem Steel Company, a corporation ofPennsylvania No Drawing. Application March 14, 1952, Serial No. 276,693

5 Claims.- (Cl. 148-63.)

This invention relates to an improved method of treating electrolytictinplate surfaces, and more particularly to a method of cleaning andmodified passivating of electrolytic tinplate produced in continuousstrip form Which may, before treating, have upon its surface deleteriousproducts'formed during, and in some cases prior to, the

enamels, and particularly phenolic type enamels, will satisfactorilyadhere, not only at the time of formation, but after extended storageperiods as Well.

Another object is to prevent yellow discoloration'of the tinplatedsurface during baking operations.

'In the manufacture of continuous electrolytic tinplate in strip form itis common practice to pass the tinplated strip thru a tin-fusingfurnace. Fusing of the coating has a manifold purpose, chief of which isthe production of a smooth surface having a bright lustre, and one whichis less susceptible to abrasion marring. When the furnace atmospheresurrounding the plated strip is of a non-protective nature,objectionable surface products may develop thru contact of the furnaceatmosphere with the strip. In some cases fluxes such as ammonium, zincor hydrogen chlorides are used prior to, or during the fusing step. Whena flux is used so as to enhance the surface lustre of the tinplate afterfusion, the flux itself may produce a film or minute spots on thesurface of the tin plate which are objectionable if they are notthoroughly removed.

In addition to the foregoing means by which deleterious substances mayform on tinplate, oxides or metal salts formed during the pickling orelectroplating operations, and present as minute surface residues on thesteel surface prior to plating, have a tendency to come to the surfaceof the plated metal during the fusing operation.

Much of the tinplate used for cans requires a coating of enamel, and oneof the commonly used enamels is the thermo-setting phenolic type. Thephenolic type of enamel is very sensitive, and will not adheresatisfactorily to a tinplate surface which has a significant quantity ofextraneous substances such as oxidation products upon it. n

I In some cases, especially when the tinplating and tinfusing areperformed as separate non-continuous steps, it is desirable to removefrom the plated strip, before fusing, products of oxidation orhydrolysisformed in the interval between plating and fusing, as well as traces ofdifficultly waterrinsable substances from theelectroplating bath. Suchcleaning may be accomplished by treating the tinplatedstrip in acathodic alkaline cleaning bath of a solution such as trisodiumphosphate or sodium carbonate or 2,775,535 Patented Dec. 25, 1956 iceother suitable solutions of sodium or potassium salts, prior topassingthe strip thru a fusion furnace.

In processing the strip, as has been previously explained, certaindetrimental impurities may accumulate on the strip. In order to insurean adequately clean and an adequately passivated tinplated strip, it isdesirable, and for most purposes necessary, to apply to the tinplate asuitable treatment after fusing. To obtain good enamel adherence, andother desirable surface qualities, after a fusing operation in whichdeleterious products may occur on the tinplate, it is required that asuitable surface treatment be employed. It is necessary not only toremove the deleterious products from the tinplate after fusing, but itis essential that some passivating treatment be applied also, so thatoxidation and its concomitant,- oxide staining, are prevented, orsufficiently retarded, while the tinplate is in storage prior toenameling.

Various methods of treating tinplate after fusion to insure good enameladherence have been tried with indifferent success. I have found thatwhen electrolytic tinplated strip is cleaned cathodically in a dilutealkaline solution just prior to fusing, and the strip is then passedthru a fusing furnace, if the strip is given a sequential treatment ofcathodic cleaning in a dilute alkaline solution and rinsed in a dilutechromic acid solution, good adherence of enamels, including thethermo-setting phenolic type, is obtained, and surface oxidation isadequately retarded, even when the strip has been in storage forrelatively long periods of time prior to enameling.

Because of the sensitivity of the phenolic enamels it is necessary notonly to select the proper solutions for postfusion treatment, but I findit mandatory to use the two stage post-fusing treatment as hereindescribed to obtain the optimum surface conditions. As a practicalmatter, when treating continuous strip, it is obvious that treating timemust be reduced to a minimum so as not to impede the progress of thestrip thru the entire sequence of finishing operations. Temperature andconcentration of the solutions must be controlled within reasonablelimits.

dilute alkaline solution such as sodium carbonate, rinsed with Water andthen passed thru a dilute chromic acid solution. The strip may then berinsed in water and dried and oiled in any conventional manner.

Bythe. above method the strip surface is denuded of impuritiesaccumulated from the plating and/or by oxidation products formed afterplating and prior to fusing, impurities which might otherwise produce anunfavorable reaction'in-the elevated temperature conditions of thefusion furnace, the post-fusion cleaningsubsequently removing anydeleterious products that may be produced on the strip while in-thefurnace and due to the accumulation on the tin surface of impuritiesoriginally on the steel surface, orjfrom fluxes used in conjunction withfusing. By Water rinsing the strip, as it leaves the postfusioncleaner,the strip enters the chromic acid rinse with an adequately cleaned tinsurface. The chromic acid, in the dilute concentration and short timeintervals used, tends to render a partial passivation of the tin surfaceso that the tinplate may be stored, even in moist atmospheres, forcommercially employed periods of time 3 without the development of anundesirable degree of surface oxidation. It has been found that completepassivation of the tinplate tends to render a poor bond between thetinplated surface and certain enamels, therefore, the partial, ormodified, passivation of the tin surface is necessary in order toprovide cleaned tinplate whichis receptive tophenolic enamels, and sothat application of the enamel results in a good bond'with the tinnedsurface. Furthermore, the clean, partially passivated tinplate doesnot'develop, in subsequent baking operations,.the yellow oxide stainnoticeable on improperly cleaned tinplate.

While sodium carbonate solution has produced good results in both thepre-fusion cleaner and the post-fusion cleaner, a solution of anyalkaline compound may be used which functions primarily to releaseappreciable quantities of sodium or potassium ions at the cathode uponelectrolysis. The pH of the solution immediately adjacent thecathodically treated metal surface must be above 7, in order to obtain aproperly cleaned tin surface. However, I have found it desirable tomaintain the pH of the solution above 9. Other compounds, or mixtures ofthe same, which may be used are sodium hydroxide, tri-sodium phosphate,potassium carbonate, potassium hydroxide, sodium bicarbonate,mono-sodium hydrogen phosphate, di-sodium hydrogen phosphate andpotassium bicarbonate. The concentration of the alkaline solutionsshould be kept quite low, in the range of from 0.5% to 5.5%, preferably1% to 3%. In the pre-cleaner, as in the post-cleaner, the currentdensity may range between 3 and 30 amp./sq."ft., while the temperatureshould range between 160 F. and 210 F. Excellent results have beenobtained when the current density has been maintained at approximatelyamp/sq. ft. at a temperature of around 180 F. The time required forcleaning in either the pre-cleaner or the post-cleaner should be, forpractical operation, from 0.5 to 3 seconds, although longer periods ofoperating time are permissible.

In partially passivating the electrotinplated strip by immersing thestrip in aqueous chromic acid solution 1 after the post-fusion cleaningoperation, it is preferable to use a chromic acid (CrOs) concentrationof from about 0.1% to 1.0%, if the strip is subsequently rinsed withwater. Generally, the immersion time for the chromic acid treatment issubstantially the same as for post-cleaning.

Another method of obtaining a tinplated strip surface having the properdegree of passivation is that of first immersing the strip in 0.1% to1.0% chromic acid and then passing the strip thru a second, dilute,chromic acid bath of less than 0.1% concentration, with no furtherrinsing.

A third alternative for chromic acid immersion is that of a one-stepimmersion in chromic acid with no further rinsing, wherein chromic acidconcentration is less than 0.1%.

The first method of partially passivating the electrotinplated stripdoes not employ a definite surface residual of chromic acid. This methodpermits optimum phenolic adhesion at some sacrifice in degree of surfacepassivity attained. The second and third methods employ minute surfaceresiduals of chromic acid which accomplish an increased degree ofsurface passivity but at some sacrifice in phenolic enamel adhesionquality.

For certain types of tinplate products, such as milk cans, where finalcoating operations, i. e., enameling, etc., are not applied to thetinplate, and thus, the passivating chromatic acid dip is unnecessary,my invention can be used' as a three-step process, wherein theelectroplated strip is cathodically cleaned in alkaline solution, passedthru the tin-fusing step and then cathodically cleaned in alkalinesolution under the controlled conditions previously described. In asituation where pro-fusion cleaning is considered unnecessary, as may bethe case when electroplating and fusion are performed as continuousin-line operations, my invention may be limited to a two-step process,i. e., the tinplate is fused and then cleaned cathodically in alkalinesolution under controlled conditions.

I claim:

1. In a method for treating electrolytic tinplate in strip form thesteps comprising heating strip to a temperature above the melting pointof tin, cleaning the strip cathodically in an aqueous alkaline solutionof sodium carbonate in which the concentration of alkaline compound isapproximately 0.5% to 5.5% at a current density of from 3 to 30 .amp./sq. ft. and at a temperature of from 160 F. to 210 F. for a period oftime in excess of 0.5 second, rinsing the strip with water and thenpassing the strip thru a chromic acid solution having a CrOconcentration of from 0.1% to 1.0% at a temperature of from F. to 200F., and rinsing the strip.

2. In a method for treating electrolytic tinplate in strip form thesteps comprising heating strip to a temperature above the melting pointof tin, cleaning the strip cathodically in an aqueous alkaline solutionof trisodium phosphate in which the concentration of alkaline compoundis approximately 0.5% to 5.5% at a current density of from 3 to 30 amp./sq. ft. and at a temperature of from F. to 210 F. for a period of timein excess of 0.5 second, rinsing the strip with water and then passingthe strip thru a chromic acid solution having a C10,; concentration offrom 0.1% to 1.0% at a temperature of from 150 F. to 200 F., and rinsingthe strip.

3. In a method for treating electrolytic tinplate in strip form thesteps comprising heating the strip to a temperature above the meltingpoint of tin, cleaning the strip cathodically in an aqueous alkalinesolution formed from an alkali metal compound of the group consisting ofsodium carbonate, sodium hydroxide, trisodium phosphate, sodiumbicarbonate, monosodium hydrogen phosphate, di-sodium hydrogenphosphate, potassium bicarbonate, potassium hydroxide, potassiumcarbonate and mixtures thereof in which the concentration of alkalimetal compound is approximately 0.5% to 5.5% at a current density offrom 3 to 30 amp/sq. ft. and at a temperature of from 160 F. to 210 F.for a period of time in excess of 0.5 second, rinsing the strip withwater and then passing the strip through a chromic acid solution havinga C1O3 concentration of from 0.1% to 1.0% and rinsing the strip.

4. In a method for treating steel having thereon a fused coating ofelectrolytically deposited tin, the steps comprising cleaning thematerial cathodically in a bath consisting essentially of an aqueoussolution of a compound of the group consisting of sodium carbonate,sodium hydroxide, trisodium phosphate, sodium bicarbonate, mono-sodiumhydrogen phosphate, di-sodium hydrogen phosphate, potassium bicarbonate,potassium hydroxide, potassium carbonate and mixtures thereof, in whichthe concentration of such compound is between 0.5% and 5.5 rinsing thematerial with water, passing the strip thru a chromic acid solutionhaving a CrOa concentration between 0.1% and 1.0% and then rinsing thestrip with Water.

5. The method of treating electrolytic tinplate in strip form whereinthe tin coating of the tinplate has been fused, which comprises cleaningthe strip cathodically in an aqueous alkaline solution formed from analkali metal compound of the group consisting of sodium carbonate,sodium hydroxide, trisodium phosphate, sodium bicarbonate, monosodiumhydrogen phosphate, di-sodium hydrogen phosphate, potassium bicarbonate,potassium hydroxide, potassium carbonate and mixtures thereof in whichthe concentration of alkali metal compound is from approximately 0.5% to5.5% at a current density of from 3 to 30 amp./ sq. ft. and at atemperature of from 160 F. to 210 F. for a period of time not less than0.5 second, rinsing the strip with water and then passing the strip thrua chromic acid solution having a CrOs concentration of from 0.1% to 1.0%and rinsing the strip.

References Cited in the file of this patent UNITED STATES PATENTS2,240,265 Nachtman Apr. 29, 1941 2,314,818 Cook et a1 Mar. 23, 19432,410,213 Herro et a1. Oct. 29, 1946 2,450,509 Glock Oct. 5, 1948 6OTHER REFERENCES 5 through 265, only pages 259 and 260 relied on.

Improved Chromating Process, by S. E. Maxon, published Dec. 13, 1943, inSteel, pages 134-136.

1. IN A METHOD FOR TREATING ELECTROLYTIC TINPLATE IN STRIP FORM THESTEPS COMPRISING HEATING STRIP TO A TEMPERATURE ABOVE THE MELTING POINTOF TIN, CLEANING THE STRIP CATHODICALLY IN AN AQUEOUS ALKALINE SOLUTIONOF SODIUM CARBONATE IN WHICH THE CONCENTRATION OF ALKALINE COMPOUND ISAPPROXIMATELY 0.5% TO 5.5% AT A CURRENT DENSITY OF FROM 3 TO 30 AMP./SQ.FT. AND AT A TEMPERATURE OF FROM 160* F. TO 210* F. FOR A PERIOD OF TIMEIN EXCESS OF 0.5 SECOND, RINSING THE STRIP WITH WATER AND THEN PASSINGTHE STRIP THRU A CHROMIC ACID SOLUTION HAVING A CRO3 CONCENTRATION OFFROM 0.1% TO 1.0% AT A TEMPERATURE OF FROM 150* F. TO 200* F., ANDRINSING THE STRIP.